Stacked white OLED having separate red, green and blue sub-elements

ABSTRACT

The present invention relates to efficient organic light emitting devices (OLEDs). More specifically, the present invention relates to white-emitting OLEDs, or WOLEDs. The devices of the present invention employ three emissive sub-elements, typically emitting red, green and blue, to sufficiently cover the visible spectrum. The sub-elements are separated by charge generating layers.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present U.S. patent application is a divisional application of U.S.patent application Ser. No. 13/124,698 filed Jun. 24, 2011, which is aU.S. National Stage Entry of international application numberPCT/US2009/062354 filed Oct. 28, 2009, which international applicationclaims priority benefit from U.S. Provisional Application No. 61/109,074filed Oct. 28, 2008, all of which are incorporated herein by referencein their entireties.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with U.S. Government support under Contract No.DE-FG02-07ER84809 awarded by the Department of Energy. The governmentmay have certain rights in this invention.

STATEMENT REGARDING JOINT RESEARCH AGREEMENT

The claimed invention was made by, on behalf of, and/or in connectionwith one or more of the following parties to a jointuniversity-corporation research agreement: Princeton University, TheUniversity of Southern California, Regents of the University ofMichigan, and Universal Display Corporation. The agreement was in effecton and before the date the claimed invention was made, and the claimedinvention was made as a result of activities undertaken within the scopeof the agreement.

FIELD OF THE INVENTION

The present invention relates to efficient organic light emittingdevices (OLEDs). More specifically, the present invention relates towhite-emitting OLEDs, or WOLEDs. The devices of the present inventionemploy three emissive sub-elements, typically emitting red, green andblue, to sufficiently cover the visible spectrum. The sub-elements areseparated by charge generating layers. This allows the construction ofbright and efficient WOLEDs that exhibit a high color rendering index.

BACKGROUND

Opto-electronic devices that make use of organic materials are becomingincreasingly desirable for a number of reasons. Many of the materialsused to make such devices are relatively inexpensive, so organicopto-electronic devices have the potential for cost advantages overinorganic devices. In addition, the inherent properties of organicmaterials, such as their flexibility, may make them well suited forparticular applications such as fabrication on a flexible substrate.Examples of organic opto-electronic devices include organic lightemitting devices (OLEDs), organic phototransistors, organic photovoltaiccells, and organic photodetectors. For OLEDs, the organic materials mayhave performance advantages over conventional materials. For example,the wavelength at which an organic emissive layer emits light maygenerally be readily tuned with appropriate dopants.

As used herein, the term “organic” includes polymeric materials as wellas small molecule organic materials that may be used to fabricateorganic opto-electronic devices. “Small molecule” refers to any organicmaterial that is not a polymer, and “small molecules” may actually bequite large. Small molecules may include repeat units in somecircumstances. For example, using a long chain alkyl group as asubstituent does not remove a molecule from the “small molecule” class.Small molecules may also be incorporated into polymers, for example as apendent group on a polymer backbone or as a part of the backbone. Smallmolecules may also serve as the core moiety of a dendrimer, whichconsists of a series of chemical shells built on the core moiety. Thecore moiety of a dendrimer may be a fluorescent or phosphorescent smallmolecule emitter. A dendrimer may be a “small molecule,” and it isbelieved that all dendrimers currently used in the field of OLEDs aresmall molecules. In general, a small molecule has a well-definedchemical formula with a single molecular weight, whereas a polymer has achemical formula and a molecular weight that may vary from molecule tomolecule. As used herein, “organic” includes metal complexes ofhydrocarbyl and heteroatom-substituted hydrocarbyl ligands.

OLEDs make use of thin organic films that emit light when voltage isapplied across the device. OLEDs are becoming an increasinglyinteresting technology for use in applications such as flat paneldisplays, illumination, and backlighting. Several OLED materials andconfigurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and5,707,745, which are incorporated herein by reference in their entirety.

OLED devices are generally (but not always) intended to emit lightthrough at least one of the electrodes, and one or more transparentelectrodes may be useful in an organic opto-electronic device. Forexample, a transparent electrode material, such as indium tin oxide(ITO), may be used as the bottom electrode. A transparent top electrode,such as disclosed in U.S. Pat. Nos. 5,703,436 and 5,707,745, which areincorporated by reference in their entireties, may also be used. For adevice intended to emit light only through the bottom electrode, the topelectrode does not need to be transparent, and may be comprised of athick and reflective metal layer having a high electrical conductivity.Similarly, for a device intended to emit light only through the topelectrode, the bottom electrode may be opaque and/or reflective. Wherean electrode does not need to be transparent, using a thicker layer mayprovide better conductivity, and using a reflective electrode mayincrease the amount of light emitted through the other electrode, byreflecting light back towards the transparent electrode. Fullytransparent devices may also be fabricated, where both electrodes aretransparent. Side emitting OLEDs may also be fabricated, and one or bothelectrodes may be opaque or reflective in such devices.

As used herein, “top” means furthest away from the substrate, while“bottom” means closest to the substrate. For example, for a devicehaving two electrodes, the bottom electrode is the electrode closest tothe substrate, and is generally the first electrode fabricated. Thebottom electrode has two surfaces, a bottom surface closest to thesubstrate, and a top surface further away from the substrate. Where afirst layer is described as “disposed over” a second layer, the firstlayer is disposed further away from substrate. There may be other layersbetween the first and second layer, unless it is specified that thefirst layer is “in physical contact with” the second layer. For example,a cathode may be described as “disposed over” an anode, even thoughthere are various organic layers in between.

As used herein, “solution processible” means capable of being dissolved,dispersed, or transported in and/or deposited from a liquid medium,either in solution or suspension form.

As used herein, and as would be generally understood by one skilled inthe art, a first “Highest Occupied Molecular Orbital” (HOMO) or “LowestUnoccupied Molecular Orbital” (LUMO) energy level is “greater than” or“higher than” a second HOMO or LUMO energy level if the first energylevel is closer to the vacuum energy level. Since ionization potentials(IP) are measured as a negative energy relative to a vacuum level, ahigher HOMO energy level corresponds to an IP having a smaller absolutevalue (an IP that is less negative). Similarly, a higher LUMO energylevel corresponds to an electron affinity (EA) having a smaller absolutevalue (an EA that is less negative). On a conventional energy leveldiagram, with the vacuum level at the top, the LUMO energy level of amaterial is higher than the HOMO energy level of the same material. A“higher” HOMO or LUMO energy level appears closer to the top of such adiagram than a “lower” HOMO or LUMO energy level.

The quality of white illumination sources can be described by a simpleset of parameters. The color of the light source is given by its CIEchromaticity coordinates x and y. The CIE coordinates are typicallyrepresented on a two dimensional plot. Monochromatic colors fall on theperimeter of the horseshoe shaped curve starting with blue in the lowerleft, running through the colors of the spectrum in a clockwisedirection to red in the lower right. The CIE coordinates of a lightsource of given energy and spectral shape will fall within the area ofthe curve. Summing light at all wavelengths uniformly gives the white orneutral point, found at the center of the diagram (CIE x,y-coordinates,0.33, 0.33). Mixing light from two or more sources gives light whosecolor is represented by the intensity weighted average of the CIEcoordinates of the independent sources. Thus, mixing light from two ormore sources can be used to generate white light. While the twocomponent and three component white light sources may appear identicalto an observer (CIE x,y-coordinates, 0.32, 0.32), they may not beequivalent illumination sources. When considering the use of these whitelight sources for illumination, the CIE color rendering index (CRI) maybe useful in addition to the CIE coordinates of the source. The CRIgives an indication of how well the light source will render colors ofobjects it illuminates. A perfect match of a given source to thestandard illuminant gives a CRI of 100. Though a CRI value of at least70 may be acceptable for certain applications, a preferred white lightsource will have a CRI of about 80 or higher.

White organic light-emitting diodes (WOLEDs) have shown their potentialas a new generation of solid-state lighting sources. However, in orderto be practical for general lighting applications, it is important toobtain high efficiency at high luminance (for example, around 1000cd/m²). Conventional WOLEDs have introduced red, green, and blue (R, G,and B) phosphorescent and/or fluorescent dopants in either a singleemission layer (EML), or multiple emissive layers that allow for excitonformation in an expanded region. For the latter structure, a suitablecombination of hosts and phosphorescent dopants can be difficult due tothe multiple constraints that are placed on the relative energies of theconstituents in these architectures.

SUMMARY OF THE INVENTION

The present invention provides a stacked OLED in which individual red(R), green (G) and blue (B) sub-elements are vertically stacked andelectrically connected by transparent charge-generating layers (CGL).The combined emission from the red, green and blue sub-elements providea white emission from the stacked device.

In one embodiment of the present invention, the stacked organic lightemitting device comprises in order a cathode; a red-emitting sub-elementcomprising an emissive layer comprising a phosphorescent red emissivematerial; a charge-generating layer; a green-emitting sub-elementcomprising an emissive layer comprising a phosphorescent green emissivematerial; a charge-generating layer; a blue-emitting sub-elementcomprising an emissive layer comprising a phosphorescent blue emissivematerial; and an anode; wherein a combined emission of the emissivematerials gives a white emission from the device.

In another embodiment of the present invention, the stacked organiclight emitting device comprises in order a cathode; a red-emittingsub-element comprising an emissive layer comprising a phosphorescent redemissive material; a charge-generating layer; a blue-emittingsub-element comprising an emissive layer comprising a phosphorescentblue emissive material; a charge-generating layer; a green-emittingsub-element comprising an emissive layer comprising a phosphorescentgreen emissive material; and an anode; wherein a combined emission ofthe emitting materials gives a white emission from the device.

In another embodiment of the present invention, the stacked organiclight emitting device comprises in a cathode; a red-emitting sub-elementcomprising an emissive layer comprising a phosphorescent red emissivematerial; a charge-generating layer; a green-emitting sub-elementcomprising an emissive layer comprising a phosphorescent green emissivematerial; a charge-generating layer; a blue-emitting sub-elementcomprising an emissive layer comprising a phosphorescent blue emissivematerial; and an anode; wherein a combined emission of the emissivematerials gives a white emission from the device; and wherein each ofthe red, green and blue sub-elements are substantially charge balanced.To achieve charge balance and high efficiency in each sub-element in thestack, different charge balancing mechanisms may be used for eachelement. These include (I) adjusting the thickness of the ETL, (2)inserting charge blocking layer(s) around the EML and (3) adjusting thethickness of the HTL. Thus, in preferred embodiments of the invention,the charge balance factor, γ, is near unity for each of the subcells inthe stacked device. Preferably, the charge balance factor for each ofthe sub-cells is from about 0.9 to 1, and more preferably from about0.95 to 1.

In preferred embodiments, each sub-element comprises a hole transportinglayer, an electron transporting layer and the emissive layer, whereinthe emissive layer is the hole transporting layer, the electrontransporting layer or a separate layer.

In a preferred embodiment of the invention, each of the emissive layersof the stacked OLED is close enough to its antinode so as to provide atleast 90% of its maximum emission.

In preferred embodiments of the invention, the charge-generating layerscomprise a material selected from MoO₃, V₂O₅, ITO, TiO₂, WO₃ and SnO₂.

It is an object of the invention to provide a stacked WOLED having ahigh efficiency. Thus, in preferred embodiments, the device has a totalmaximum external quantum efficiency of at least about 30%.

It is an object of the invention to provide a stacked WOLED having ahigh efficiency at high brightness. Thus, in preferred embodiments, thedevice has a total external quantum efficiency of at least about 28% ata brightness of about 1000 cd/m².

It is a further object of the invention to provide a stacked WOLEDhaving a white emission that is suitable for indoor lightingapplications. Thus, in preferred embodiments, the device emits lighthaving CIE coordinates of X=0.37±0.08, and Y=0.37±0.08.

It is a further object of the invention to provide a stacked WOLEDhaving a high CRI. Thus, in preferred embodiment, the device emits lighthaving a CRI of at least 70, and more preferably at least 75.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a stacked organic light emitting device having individualsub-elements separated by charge generating layer, as well as otherlayers.

FIG. 2 shows a sub-element having an emissive layer, as well as otherlayers.

FIG. 3 shows schematic device structures for the white-emitting OLEDs ofthe present invention having red, green and blue sub-elements separatedby charge-generating layers.

FIG. 4 shows the schematic energy level diagrams of the R-G-B WOLED ofExample 1 consisting of red, green, and blue OLED sub-elements.

FIG. 5 shows the emission spectra for each sub-element as a function ofits position in the stacked OLED structure.

FIG. 6 shows the measured and simulated electroluminescence spectra forthe device of Example 1 at current densities of J=1, 10, 100 mA/cm². Thesimulated spectra are based on cavity enhancement and extractionefficiencies by fitting ratios of photos generated from R, G, and Bcells.

FIG. 7 shows the plots of current density versus voltage characteristicsof the stacked OLED of Example 1 and control devices comprising theseparate individual sub-elements.

FIG. 8 shows the total external quantum efficiency as a function ofcurrent density for the R-G-B SOLED of Example 1 and for the individualred, green and blue control devices as a reference.

FIG. 9 shows the total power efficiency as a function of current densityfor the R-G-B SOLED of Example 1 and for the individual red, green andblue control devices as a reference. The arrow indicates values atbrightness of 1000 cd/m².

FIG. 10 shows the emission spectrum of the optically optimized stackeddevices having the sub-element order B-G-R (solid line) and R-G-B(dashed line).

FIG. 11 shows the power extracted, considering microcavity effect aswell as extraction efficiency, as a function of position and wavelengthfor the stacked OLEDs.

FIG. 12 shows (a) the proposed energy-level diagram of athree-subelement tris-(phenylpyridine)iridium (Ir(ppy)₃) SOLED; and (b)the energy level of a CGL in a proposed thermally assisted tunnelingmodel, where φ_(t) is the trap level with respect to MoO₃ valence bandmaximum, and φ_(B) is the tunneling barrier. Holes (open circle) andelectrons (solid circle) are then dissociated under the electric field,resulting in current density J_(h,CGL) and J_(e,CGL), respectively.

FIG. 13 shows the proposed energy-level diagrams of the (a) electron-,and (b) hole-only devices.

FIG. 14 shows (a) the room-temperature J-V characteristics of theelectron-only devices with MoO₃ of the thickness 50 Å (square), 100 Å(circle), and 200 Å (triangle); and (b) the J-V characteristics of thehole-only device without MoO₃ (Al 500 Å/Li:BCP 100 Å/NPD 400 Å/MoO₃ 50Å/Al 500 Å) under 159K (open square) and 296K (open circle).

FIG. 15 shows (a) the C-V characteristics; (b) the calculated depletionwidths of the electron-only devices with MoO₃ of the thickness 50 Å(square), 100 Å (circle), and 200 Å (triangle) at the frequency of 200Hz; and (c) the calculated depletion widths of the electron-only deviceswith 100-A-thick MoO₃ with Li:BCP in a 1:10 molar ratio (circle), andLi:BCP in a 1:1 molar ratio (triangle), and without Li doping (square).

FIG. 16 shows the current density (J) vs. inverse electric field (E) forelectron-only devices with MoO₃ thickness of (a) 50 Å, (b) 100 Å and (c)200 Å under temperatures varied from 159K to 296K. The solid lines arefits according to Eqs. (1) and (2) to yield both the tunneling and trapenergy barriers listed in Table I.

FIG. 17 shows the current density (J) vs. 1000/T, where T is thetemperature, for electron-only devices at an applied electric fieldE=2.0×10⁷ V/cm, except for the device with 200 Å-thick MoO₃, in whichE=2.6×10⁷ V/cm is used. Solid line fits yield the trap energy level,φ_(t), listed in Table I.

FIG. 18 shows the current density (J) vs. inverse electric field (E) forhole-only devices with MoO₃ thickness of (a) 50 Å, (b) 100 Å, and (c)200 Å under temperatures varied from 180K to 296K. The solid lines arefits according to Eqs. (1) and (2) to yield both the tunneling and trapenergy barriers listed in Table I.

FIG. 19 shows the current density (J) vs. 1000/T, where T is thetemperature, for hole-only devices at an applied electric fieldE=1.6×10⁷ V/cm. Solid line fits yield the trap energy level, φ_(t),listed in Table I.

FIG. 20 shows (a) the schematic of the currents that establish chargebalance in a SOLED with three subelements, wherein the directions ofcurrent densities are indicated by arrows and parasitic leakage currentsare indicated by dashed lines; (b) the external quantum efficiencies and(c) power efficiencies of Cell-L (open square), Cell-M (invertedtriangle), Cell-R (open circle), and the control device (triangle).

FIG. 21 shows (a) the external quantum efficiencies and (b) the powerefficiencies of Cell-R with various BCP thicknesses.

FIG. 22 shows (a) the external quantum efficiencies and (b) the powerefficiencies of G-G-G SOLEDs with various BCP thicknesses in Cell-R.

DETAILED DESCRIPTION

The present invention provides a stacked OLED in which individual red(R), green (G) and blue (B) sub-elements are vertically stacked andelectrically connected by transparent charge-generating layers (CGL).

Generally, an OLED comprises at least one organic layer disposed betweenand electrically connected to an anode and a cathode. When a current isapplied, the anode injects holes and the cathode injects electrons intothe organic layer(s). The injected holes and electrons each migratetoward the oppositely charged electrode. When an electron and holelocalize on the same molecule, an “exciton,” which is a localizedelectron-hole pair having an excited energy state, is formed. Light isemitted when the exciton relaxes via a photoemissive mechanism. In somecases, the exciton may be localized on an excimer or an exciplex.Non-radiative mechanisms, such as thermal relaxation, may also occur,but are generally considered undesirable.

The initial OLEDs used emissive molecules that emitted light from theirsinglet states (“fluorescence”) as disclosed, for example, in U.S. Pat.No. 4,769,292, which is incorporated by reference in its entirety.Fluorescent emission generally occurs in a time frame of less than 10nanoseconds.

More recently, OLEDs having emissive materials that emit light fromtriplet states (“phosphorescence”) have been demonstrated. Baldo et al.,“Highly Efficient Phosphorescent Emission from OrganicElectroluminescent Devices,” Nature, vol. 395, 151-154, 1998;(“Baldo-I”) and Baldo et al., “Very high-efficiency green organiclight-emitting devices based on electrophosphorescence,” Appl. Phys.Lett., vol. 75, No. 1, 4-6 (1999) (“Baldo-II”), which are incorporatedby reference in their entireties. Phosphorescence may be referred to asa “forbidden” transition because the transition requires a change inspin states, and quantum mechanics indicates that such a transition isnot favored. As a result, phosphorescence generally occurs in a timeframe exceeding at least 10 nanoseconds, and typically greater than 100nanoseconds. If the natural radiative lifetime of phosphorescence is toolong, triplets may decay by a non-radiative mechanism, such that nolight is emitted. Organic phosphorescence is also often observed inmolecules containing heteroatoms with unshared pairs of electrons atvery low temperatures. 2,2′-bipyridine is such a molecule. Non-radiativedecay mechanisms are typically temperature dependent, such that anorganic material that exhibits phosphorescence at liquid nitrogentemperatures typically does not exhibit phosphorescence at roomtemperature. But, as demonstrated by Baldo, this problem may beaddressed by selecting phosphorescent compounds that do phosphoresce atroom temperature. Representative emissive layers include doped orun-doped phosphorescent organometallic materials such as disclosed inU.S. Pat. Nos. 6,303,238 and 6,310,360; U.S. Patent ApplicationPublication Nos. 2002/0034656; 2002/0182441; 2003/0072964; andWO-02/074015.

Generally, the excitons in an OLED are believed to be created in a ratioof about 3:1, i.e., approximately 75% triplets and 25% singlets. See,Adachi et al., “Nearly 100% Internal Phosphorescent Efficiency In AnOrganic Light Emitting Device,” J. Appl. Phys., 90, 5048 (2001), whichis incorporated by reference in its entirety. In many cases, singletexcitons may readily transfer their energy to triplet excited states via“intersystem crossing,” whereas triplet excitons may not readilytransfer their energy to singlet excited states. As a result, 100%internal quantum efficiency is theoretically possible withphosphorescent OLEDs. In a fluorescent device, the energy of tripletexcitons is generally lost to radiationless decay processes that heat-upthe device, resulting in much lower internal quantum efficiencies. OLEDsutilizing phosphorescent materials that emit from triplet excited statesare disclosed, for example, in U.S. Pat. No. 6,303,238, which isincorporated by reference in its entirety.

Phosphorescence may be preceded by a transition from a triplet excitedstate to an intermediate non-triplet state from which the emissive decayoccurs. For example, organic molecules coordinated to lanthanideelements often phosphoresce from excited states localized on thelanthanide metal. However, such materials do not phosphoresce directlyfrom a triplet excited state but instead emit from an atomic excitedstate centered on the lanthanide metal ion. The europium diketonatecomplexes illustrate one group of these types of species.

Phosphorescence from triplets can be enhanced over fluorescence byconfining, preferably through bonding, the organic molecule in closeproximity to an atom of high atomic number. This phenomenon, called theheavy atom effect, is created by a mechanism known as spin-orbitcoupling. Such a phosphorescent transition may be observed from anexcited metal-to-ligand charge transfer (MLCT) state of anorganometallic molecule such as tris(2-phenylpyridine)iridium(III).

As used herein, the term “triplet energy” refers to an energycorresponding to the highest energy feature discernable in thephosphorescence spectrum of a given material. The highest energy featureis not necessarily the peak having the greatest intensity in thephosphorescence spectrum, and could, for example, be a local maximum ofa clear shoulder on the high energy side of such a peak.

The term “organometallic” as used herein is as generally understood byone of ordinary skill in the art and as given, for example, in“Inorganic Chemistry” (2nd Edition) by Gary L. Miessler and Donald A.Tarr, Prentice Hall (1998). Thus, the term organometallic refers tocompounds which have an organic group bonded to a metal through acarbon-metal bond. This class does not include per se coordinationcompounds, which are substances having only donor bonds fromheteroatoms, such as metal complexes of amines, halides, pseudohalides(CN, etc.), and the like. In practice organometallic compounds generallycomprise, in addition to one or more carbon-metal bonds to an organicspecies, one or more donor bonds from a heteroatom. The carbon-metalbond to an organic species refers to a direct bond between a metal and acarbon atom of an organic group, such as phenyl, alkyl, alkenyl, etc.,but does not refer to a metal bond to an “inorganic carbon,” such as thecarbon of CN or CO.

FIG. 1 shows a stacked organic light emitting device 100. The figuresare not necessarily drawn to scale. Device 100 may include a substrate110, an anode 120, a hole injection layer 130, OLED sub-elements 140,charge-generating layers 150, an electron injection layer 160, aprotective layer 170, and a cathode 180. Cathode 180 may be a compoundcathode having a first conductive layer 182 and a second conductivelayer 184. Device 100 may be fabricated by depositing the layersdescribed, in order.

FIG. 2 shows a sub-element 140. Each OLED sub-element 140 may include ahole injection layer 220, a hole transport layer 225, an electronblocking layer 230, an emissive layer 235, a hole blocking layer 240, anelectron transport layer 245, an electron injection layer 250, aprotective layer 255. Each separate sub-element may have a differentlayer structure from the other sub-elements and/or may be comprised ofdifferent materials.

Substrate 110 may be any suitable substrate that provides desiredstructural properties. Substrate 110 may be flexible or rigid. Substrate110 may be transparent, translucent or opaque. Plastic and glass areexamples of preferred rigid substrate materials. Plastic and metal foilsare examples of preferred flexible substrate materials. Substrate 110may be a semiconductor material in order to facilitate the fabricationof circuitry. For example, substrate 110 may be a silicon wafer uponwhich circuits are fabricated, capable of controlling OLEDs subsequentlydeposited on the substrate. Other substrates may be used. The materialand thickness of substrate 110 may be chosen to obtain desiredstructural and optical properties.

Anode 120 may be any suitable anode that is sufficiently conductive totransport holes to the organic layers. The material of anode 120preferably has a work function higher than about 4 eV (a “high workfunction material”). Preferred anode materials include conductive metaloxides, such as indium tin oxide (ITO) and indium zinc oxide (IZO),aluminum zinc oxide (AlZnO), and metals. Anode 120 (and substrate 110)may be sufficiently transparent to create a bottom-emitting device. Apreferred transparent substrate and anode combination is commerciallyavailable ITO (anode) deposited on glass or plastic (substrate). Aflexible and transparent substrate-anode combination is disclosed inU.S. Pat. Nos. 5,844,363 and 6,602,540 B2, which are incorporated byreference in their entireties. Anode 120 may be opaque and/orreflective. A reflective anode 120 may be preferred for sometop-emitting devices, to increase the amount of light emitted from thetop of the device. The material and thickness of anode 120 may be chosento obtain desired conductive and optical properties. Where anode 120 istransparent, there may be a range of thickness for a particular materialthat is thick enough to provide the desired conductivity, yet thinenough to provide the desired degree of transparency. Other anodematerials and structures may be used.

Generally, injection layers are comprised of a material that may improvethe injection of charge carriers from one layer, such as an electrode ora charge generating layer, into an adjacent organic layer. Injectionlayers may also perform a charge transport function. In device 100, holeinjection layer 130 may be any layer that improves the injection ofholes from anode 120 into an adjacent organic layer. CuPc is an exampleof a material that may be used as a hole injection layer from an ITOanode 120, and other anodes. In device 100, electron injection layer 160may be any layer that improves the injection of electrons into anadjacent organic layer. LiF/Al is an example of a material that may beused as an electron injection layer into an electron transport layerfrom an adjacent layer. Other materials or combinations of materials maybe used for injection layers. Depending upon the configuration of aparticular device, injection layers may be disposed at locationsdifferent than those shown in device 100. More examples of injectionlayers are provided in U.S. patent application Ser. No. 09/931,948 to Luet al., which is incorporated by reference in its entirety. A holeinjection layer may comprise a solution deposited material, such as aspin-coated polymer, e.g., PEDOT:PSS, or it may be a vapor depositedsmall molecule material, e.g., CuPc or MTDATA.

A hole injection layer (HIL) may planarize or wet the surface of theanode or a charge-generating layer so as to provide efficient holeinjection from the anode or charge-generating layer into the holeinjecting material. A hole injection layer may also have a chargecarrying component having HOMO (Highest Occupied Molecular Orbital)energy levels that favorably match up, as defined by theirherein-described relative ionization potential (IP) energies, with theadjacent anode layer on one side of the HIL and a hole transportinglayer on the opposite side of the HIL. The “charge carrying component”is the material responsible for the HOMO energy level that actuallytransports holes. This component may be the base material of the HIL, orit may be a dopant. Using a doped HIL allows the dopant to be selectedfor its electrical properties, and the host to be selected formorphological properties such as wetting, flexibility, toughness, etc.Preferred properties for the HIL material are such that holes can beefficiently injected from the anode into the HIL material. Inparticular, the charge carrying component of the HIL preferably has anIP not more than about 0.7 eV greater that the IP of the anode material.More preferably, the charge carrying component has an IP not more thanabout 0.5 eV greater than the anode material. Similar considerationsapply to any layer into which holes are being injected. HIL materialsare further distinguished from conventional hole transporting materialsthat are typically used in the hole transporting layer of an OLED inthat such HIL materials may have a hole conductivity that issubstantially less than the hole conductivity of conventional holetransporting materials. The thickness of the HIL of the presentinvention may be thick enough to help planarize or wet the surface ofthe anode layer or charge-generating layer. For example, an HILthickness of as little as 10 nm may be acceptable for a very smoothanode surface. However, since anode surfaces tend to be very rough, athickness for the HIL of up to 50 nm may be desired in some cases.

The stacked OLED 100 includes three individual sub-elements 140. Eachsub-element preferably emits a different primary color. Thus, thestacked device preferably comprises a red-emitting sub-element, agreen-emitting sub-element and a blue-emitting sub-element. Ideally thecombined emission of the individual sub-elements gives a white emissionfrom the device.

A sub-element comprises at least one organic layer which is an emissivelayer—i.e., the layer is capable of emitting light when a voltage isapplied across the stacked device. The emissive layer comprises aphosphorescent emissive material, preferably as a dopant in a hostmaterial. In more preferred device structures, each sub-elementcomprises at least two layers, one which is an electron transportinglayer and one which is a hole transporting layer. In this embodiment,either the electron transporting layer or the hole transporting layermay be the emissive layer. In particularly preferred embodiments, theelectron transporting layer is the emissive layer. In other preferreddevice structures, the sub-element comprises at least three layers—anelectron transporting layer, an emissive layer and a hole transportinglayer. In the embodiments having such a separate emissive layer, theemissive layer may be primarily conduct electrons or holes. Additionallayers may be added to a sub-element. FIG. 2 shows a sub-element 140.Each OLED sub-element 140 may include a hole injection layer 220, a holetransport layer 225, an electron blocking layer 230, an emissive layer235, a hole blocking layer 240, an electron transport layer 245, anelectron injection layer 250, a protective layer 255. Each OLEDsub-element 140 in the stacked OLED 100 may have the same layerstructure or different layer structure from the other sub-elements.

The charge-generating layers 150 are layers that injects charge carriersinto the adjacent layer(s) but do not have a direct external connection.The charge-generating layers 150 separate the sub-elements 140 of thestacked OLED. Each of the charge-generating layers 150 may be composedof the same material(s), or many have different compositions. When avoltage is applied across the stacked OLED having a charge-generatinglayer, the charge-generating layer may inject holes into the organicphosphorescent sub-element on the on the cathode side of thecharge-generating layer, and electrons into the organic phosphorescentsub-element on the anode side. As will be understood by one skilled inthe art, the “anode side” of a layer or device refers to the side of thelayer or device at which holes are expected to enter the layer ordevice. Similarly, a “cathode side” refers to the side of the layer ordevice to which electrons are expected to enter the layer or device.

Each charge-generating layer may be formed by the contact of dopedn-type (Li, Cs, Mg, etc. doped) layer with a p-type (metal oxides,F4-TCNQ, etc.) layer. In preferred embodiments, the doped n-type layermay be selected from an alkali metal or alkaline earth metal dopedorganic layer, such as Li doped BCP or Mg doped Alq₃, with Li doped BCPbeing preferred. In other preferred embodiments, the charge-generatinglayers comprise an inorganic material selected from stable metal oxides,including MoO₃, V₂O₅, ITO, TiO₂, WO₃ and SnO₂. In particularly preferredembodiments of the invention, the charge-generating layers employ alayer of MoO₃ or V₂O₅, with MoO₃ being most preferred.

A protective layer may be used to protect underlying layers duringsubsequent fabrication processes. For example, the processes used tofabricate metal or metal oxide top electrodes may damage organic layers,and a protective layer may be used to reduce or eliminate such damage.In device 100, protective layer 170 may reduce damage to underlyingorganic layers during the fabrication of cathode 180. Preferably, aprotective layer has a high carrier mobility for the type of carrierthat it transports (electrons in device 100), such that it does notsignificantly increase the operating voltage of device 100. CuPc, BCP,and various metal phthalocyanines are examples of materials that may beused in protective layers. Other materials or combinations of materialsmay be used. The thickness of protective layer 170 is preferably thickenough that there is little or no damage to underlying layers due tofabrication processes that occur after organic protective layer 160 isdeposited, yet not so thick as to significantly increase the operatingvoltage of device 100. Protective layer 170 may be doped to increase itsconductivity. For example, a CuPc or BCP protective layer may be dopedwith Li. A more detailed description of protective layers may be foundin U.S. patent application Ser. No. 09/931,948 to Lu et al., which isincorporated by reference in its entirety.

Cathode 180 may be any suitable material or combination of materialsknown to the art, such that cathode 180 is capable of conductingelectrons and injecting them into the organic layers of device 100.Cathode 180 may be transparent or opaque, and may be reflective. Metalsand metal oxides are examples of suitable cathode materials. Cathode 180may be a single layer, or may have a compound structure. FIG. 1 shows acompound cathode 180 having a thin metal layer 182 and a thickerconductive metal oxide layer 184. In a compound cathode, preferredmaterials for the thicker layer 164 include ITO, IZO, and othermaterials known to the art. U.S. Pat. Nos. 5,703,436, 5,707,745,6,548,956 B2 and 6,576,134 B2, which are incorporated by reference intheir entireties, disclose examples of cathodes including compoundcathodes having a thin layer of metal such as Mg:Ag with an overlyingtransparent, electrically-conductive, sputter-deposited ITO layer. Thepart of cathode 180 that is in contact with the underlying organiclayer, whether it is a single layer cathode 180, the thin metal layer182 of a compound cathode, or some other part, is preferably made of amaterial having a work function lower than about 4 eV (a “low workfunction material”). Other cathode materials and structures may be used.

Hole transport layer 225 includes a material capable of transportingholes. Hole transport layer 225 may be intrinsic (undoped), or doped.Doping may be used to enhance conductivity. α-NPD and TPD are examplesof intrinsic hole transport layers. An example of a p-doped holetransport layer is m-MTDATA doped with F4-TCNQ at a molar ratio of 50:1,as disclosed in United States Patent Application Publication No.2003-0230980 to Forrest et al., which is incorporated by reference inits entirety. Preferred hole transporting compounds include aromatictertiary amines, including but not limited to α-NPD, TPD, MTDATA, andHMTPD. Other hole transport layers may be used.

Emissive layer 235 includes an organic material capable of emittinglight when a current is passed between anode 120 and cathode 180.Preferably, emissive layer 235 contains a phosphorescent emissivematerial. Phosphorescent materials are preferred because of the higherluminescent efficiencies associated with such materials. Emissive layer235 may also comprise a host material capable of transporting electronsand/or holes, doped with an emissive material that may trap electrons,holes, and/or excitons, such that excitons relax from the emissivematerial via a photoemissive mechanism. Emissive layer 235 may comprisea single material that combines transport and emissive properties.Whether the emissive material is a dopant or a major constituent,emissive layer 235 may comprise other materials, such as dopants thattune the emission of the emissive material. Emissive layer 235 mayinclude a plurality of emissive materials capable of in combination,emitting a desired spectrum of light. Examples of phosphorescentemissive materials include Ir(ppy)₃. Examples of host materials includeAlq₃, CBP and mCP. Examples of emissive and host materials are disclosedin U.S. Pat. No. 6,303,238 to Thompson et al., which is incorporated byreference in its entirety. Emissive material may be included in emissivelayer 235 in a number of ways. For example, an emissive small moleculemay be incorporated into a polymer. This may be accomplished by severalways: by doping the small molecule into the polymer either as a separateand distinct molecular species; or by incorporating the small moleculeinto the backbone of the polymer, so as to form a co-polymer; or bybonding the small molecule as a pendant group on the polymer. Otheremissive layer materials and structures may be used. For example, asmall molecule emissive material may be present as the core of adendrimer.

Many useful emissive materials include one or more ligands bound to ametal center. A ligand may be referred to as “photoactive” if itcontributes directly to the photoactive properties of an organometallicemissive material. A “photoactive” ligand may provide, in conjunctionwith a metal, the energy levels from which and to which an electronmoves when a photon is emitted. Other ligands may be referred to as“ancillary.” Ancillary ligands may modify the photoactive properties ofthe molecule, for example by shifting the energy levels of a photoactiveligand, but ancillary ligands do not directly provide the energy levelsinvolved in light emission. A ligand that is photoactive in one moleculemay be ancillary in another. These definitions of photoactive andancillary are intended as non-limiting theories.

Electron transport layer 245 may include a material capable oftransporting electrons. Electron transport layer 245 may be intrinsic(undoped), or doped. Doping may be used to enhance conductivity. Alq₃ isan example of an intrinsic electron transport layer. An example of ann-doped electron transport layer is BPhen doped with Li at a molar ratioof 1:1, as disclosed in United States Patent Application Publication No.2003-02309890 to Forrest et al., which is incorporated by reference inits entirety. Other electron transport layers may be used.

The charge carrying component of the electron transport layer may beselected such that electrons can be efficiently injected from thecathode into the LUMO (Lowest Unoccupied Molecular Orbital) energy levelof the electron transport layer. The “charge carrying component” is thematerial responsible for the LUMO energy level that actually transportselectrons. This component may be the base material, or it may be adopant. The LUMO energy level of an organic material may be generallycharacterized by the electron affinity of that material and the relativeelectron injection efficiency of a cathode may be generallycharacterized in terms of the work function of the cathode material.This means that the preferred properties of an electron transport layerand the adjacent cathode may be specified in terms of the electronaffinity of the charge carrying component of the ETL and the workfunction of the cathode material. In particular, so as to achieve highelectron injection efficiency, the work function of the cathode materialis preferably not greater than the electron affinity of the chargecarrying component of the electron transport layer by more than about0.75 eV, more preferably, by not more than about 0.5 eV. Similarconsiderations apply to any layer into which electrons are beinginjected.

Blocking layers may be used to reduce the number of charge carriers(electrons or holes) and/or excitons that leave the emissive layer. Anelectron blocking layer 230 may be disposed between emissive layer 235and the hole transport layer 225, to block electrons from leavingemissive layer 235 in the direction of hole transport layer 225.Similarly, a hole blocking layer 240 may be disposed between emissivelayer 235 and electron transport layer 245, to block holes from leavingemissive layer 235 in the direction of electron transport layer 245.Blocking layers may also be used to block excitons from diffusing out ofthe emissive layer. The theory and use of blocking layers is describedin more detail in U.S. Pat. No. 6,097,147 and United States PatentApplication Publication No. 2003-02309890 to Forrest et al, which areincorporated by reference in their entireties.

As used herein, and as would be understood by one skilled in the art,the term “blocking layer” means that the layer provides a barrier thatsignificantly inhibits transport of charge carriers and/or excitonsthrough the device, without suggesting that the layer necessarilycompletely blocks the charge carriers and/or excitons. The presence ofsuch a blocking layer in a device may result in substantially higherefficiencies as compared to a similar device lacking a blocking layer.Also, a blocking layer may be used to confine emission to a desiredregion of an OLED.

In alternative embodiments, the OLED of FIG. 1 may in the form of an“inverted” OLED, in which the substrate is adjacent to the cathoderather than the anode. Such an inverted device may be fabricated bydepositing the layers described, in order on the substrate.

The simple layered structure illustrated in FIGS. 1 and 2 is provided byway of non-limiting example, and it is understood that embodiments ofthe invention may be used in connection with a wide variety of otherstructures. The specific materials and structures described areexemplary in nature, and other materials and structures may be used.Functional OLEDs may be achieved by combining the various layersdescribed in different ways, or layers may be omitted entirely, based ondesign, performance, and cost factors. Thus, certain layers may combinethe function of two or more of the layers in a single layer. Otherlayers not specifically described may also be included. Materials otherthan those specifically described may be used. Although many of theexamples provided herein describe various layers as comprising a singlematerial, it is understood that combinations of materials, such as amixture of host and dopant, or more generally a mixture, may be used.Also, the layers may have various sublayers. The names given to thevarious layers herein are not intended to be strictly limiting. Forexample, a hole transport layer may both transport holes and injectsholes into an emissive layer, and may be described as a hole transportlayer or a hole injection layer. In one embodiment, an OLED may bedescribed as having an “organic layer” disposed between a cathode and ananode. This organic layer may comprise a single layer, or may furthercomprise multiple layers of different organic materials as described,for example, with respect to FIGS. 1 and 2.

Structures and materials not specifically described may also be used,such as OLEDs comprised of polymeric materials (PLEDs) such as disclosedin U.S. Pat. No. 5,247,190, Friend et al., which is incorporated byreference in its entirety. By way of further example, OLEDs having asingle organic layer may be used. OLEDs may be stacked, for example asdescribed in U.S. Pat. No. 5,707,745 to Forrest et al, which isincorporated by reference in its entirety. The OLED structure maydeviate from the simple layered structure illustrated in FIGS. 1 and 2.For example, the substrate may include an angled reflective surface toimprove outcoupling, such as a mesa structure as described in U.S. Pat.No. 6,091,195 to Forrest et al, and/or a pit structure as described inU.S. Pat. No. 5,834,893 to Bulovic et al., which are incorporated byreference in their entireties.

Unless otherwise specified, any of the layers of the various embodimentsmay be deposited by any suitable method. For the organic layers,preferred methods include thermal evaporation, ink-jet, such asdescribed in U.S. Pat. Nos. 6,013,982 and 6,087,196, which areincorporated by reference in their entireties, organic vapor phasedeposition (OVPD), such as described in U.S. Pat. No. 6,337,102 toForrest et al., which is incorporated by reference in its entirety, anddeposition by organic vapor jet printing (OVJP), such as described inU.S. patent application Ser. No. 10/233,470, which is incorporated byreference in its entirety. Other suitable deposition methods includespin coating and other solution based processes. Solution basedprocesses are preferably carried out in nitrogen or an inert atmosphere.For the other layers, preferred methods include thermal evaporation.Preferred patterning methods include deposition through a mask, coldwelding such as described in U.S. Pat. Nos. 6,294,398 and 6,468,819,which are incorporated by reference in their entireties, and patterningassociated with some of the deposition methods such as ink jet and OVJP.Other methods may also be used. The materials to be deposited may bemodified to make them compatible with a particular deposition method.For example, substituents such as alkyl and aryl groups, branched orunbranched, and preferably containing at least 3 carbons, may be used insmall molecules to enhance their ability to undergo solution processing.Substituents having 20 carbons or more may be used, and 3-20 carbons isa preferred range. Materials with asymmetric structures may have bettersolution processibility than those having symmetric structures, becauseasymmetric materials may have a lower tendency to recrystallize.Dendrimer substituents may be used to enhance the ability of smallmolecules to undergo solution processing.

The molecules disclosed herein may be substituted in a number ofdifferent ways without departing from the scope of the invention. Forexample, substituents may be added to a compound having three bidentateligands, such that after the substituents are added, one or more of thebidentate ligands are linked together to form, for example, atetradentate or hexadentate ligand. Other such linkages may be formed.It is believed that this type of linking may increase stability relativeto a similar compound without linking, due to what is generallyunderstood in the art as a “chelating effect.”

Devices fabricated in accordance with embodiments of the invention maybe incorporated into a wide variety of consumer products, including flatpanel displays, computer monitors, televisions, billboards, lights forinterior or exterior illumination and/or signaling, heads up displays,fully transparent displays, flexible displays, laser printers,telephones, cell phones, personal digital assistants (PDAs), laptopcomputers, digital cameras, camcorders, viewfinders, micro-displays,vehicles, a large area wall, theater or stadium screen, or a sign.Various control mechanisms may be used to control devices fabricated inaccordance with the present invention, including passive matrix andactive matrix. Many of the devices are intended for use in a temperaturerange comfortable to humans, such as 18 degrees C. to 30 degrees C., andmore preferably at room temperature (20-25 degrees C.).

In preferred embodiments, each sub-element comprises a hole transportinglayer, an electron transporting layer and an emissive layer, wherein theemissive layer is the hole transporting layer, the electron transportinglayer or a separate layer.

The red-emitting sub-element comprises an emissive layer that includes aphosphorescent red emissive material. Phosphorescent red emittingmaterials are known in the art and includesIr(III)-bis-(2-phenylquinolyl-N,C^(2′))-acetylacetonate (PQIr).

The green-emitting sub-element comprising an emissive layer comprising aphosphorescent green emissive material. Phosphorescent green emittingmaterials are known in the art and include tris-(phenylpyridine) iridium(Ir(ppy)₃).

The blue-emitting sub-element comprises an emissive layer that includesa phosphorescent blue emissive material. Phosphorescent blue emittingmaterials are known in the art and includebis-(4′,6′-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6).When a phosphorescent blue-emitting dopant is used, the host may bepreferably selected from a high energy gap host, or a “wide-gap” hostmaterial. Wide-gap hosts have an energy gap that is greater than about3.0 eV, and preferably the energy gap is about 3.2 eV or greater, and anenergy gap of about 3.5 eV or greater may be particularly preferred. Thewide gap host material may be selected from materials disclosed in U.S.published application No. 2004/0209116, which is incorporated herein byreference in its entirety. A charge-carrying dopant may be employed inaddition to the emissive material in the emissive layer.

In achieving the high efficiency and balanced white emission from thestacked devices of the present invention the order of the red, blue andgreen sub-elements in the stacked device may be an importantconsideration. For balanced emission intensities from each OLEDsub-element, it may be important to control the optical interference andto optimize the weak microcavity effects in the stacked structure inorder to achieve the desired white color performance. The extractionefficiencies of the red, green and blue sub-units in the stacked OLEDmay be obtained by Cavity Modeling Framework (CAMFR), software based ona combination of eigenmode expansion and advanced boundary condition. Byvarying the orders of red, green and blue sub-units and the thickness oforganic layers, the simulation shows that the power extracted into airis a function of both the wavelength and the source position (i.e., top,middle, and bottom, with the bottom sub-element closest to theITO/substrate and the top sub-element closest to the cathode). Theresults indicate that red, green and blue sub-elements, arranged indifferent orders, have different extraction efficiencies and thus yielddifferent color temperature and color rending indices (CRI) of thestacked device with other parameters staying the same.

In certain preferred embodiments of the invention, the red-emittingsub-element is the top sub-element in the stacked WOLED. In oneembodiment, the red-emitting sub-element is the top sub-element, thegreen-emitting sub-element is the middle sub-element and theblue-emitting sub-element is the bottom sub-element (see FIG. 3A). Inanother embodiment, the red-emitting sub-element is the top sub-element,the blue-emitting sub-element is the middle sub-element and thegreen-emitting sub-element is the bottom sub-element (see FIG. 3B).

With a particular optimized RGB order, the light output can be furtheroptimized by adjusting the location of each emitter relative to themetal electrode. With the emitter closer to one of its optical antinodes(emitter to cathode round trip phase change equal (2n+1)π with n=1, 2,etc.), the corresponding emission can be boosted, and vice versa. Oneway to adjust the emitter location is to add a layer with high carriermobility in sub-elements, or to adjust the layer thickness of one orlayers of the sub-element. Preferably, the emissive layers are withinabout 20% of the distance to an antinode for that emitter. Thus, inpreferred embodiments at least two of the emissive layers, and morepreferably all of the emissive layers, are close enough to an antinodeso as to provide at least 90% of its maximum emission, and morepreferably at least 95% of its maximum emission.

A further consideration may be the injection efficiency for a chargecarrier from an electrode or a charge-generating layer into the adjacentsub-element. There may be a tradeoff between good electrical injectionand good color rendition. Thus, for example, in order to maximize theCRI and CIE parameters of the white emission from the device, the orderof the emissive sub-elements may be important. In certain embodiments ofthe invention, a lower emission from the green sub-element relative tothe red and blue sub-elements is acceptable, or even preferred, whenobtaining a balanced white emission. In this embodiment, the greensub-element is preferably the middle sub-element, as the injectionefficiency from the two adjacent charge-generating layers may be lessefficient than the injection from an adjacent electrode.

Charge injection from the compound CGL may be modeled based on atwo-step process consisting of tunneling-assisted thermionic emissionover an injection barrier of (1.2±0.2)eV and a trap level due to oxygenvacancies at (0.06±0.01)eV above the MoO₃ valence band edge.

Without being limited by theory, it is believed that electron injectionoccurs via thermionically excited electrons into traps located atenergy, φ_(t), above the metal oxide (e.g., MoO₃, etc.) valance bandmaximum, as shown in FIG. 12 b. This is followed by field-assistedtunneling through the thin depletion region of the adjacent, dopedorganic layer. At applied voltage, V, the electron (J_(e,CGL)) and hole(J_(h,CGL)) current densities in the CGL interface region in FIG. 12follow:J _(e,CGL) =J _(h,CGL) =qv _(e) N _(t) fP(V)  (1)where

$f = {1/( {1 + {\exp\lbrack \frac{q\;\varphi_{t}}{kT} \rbrack}} )}$is me Fermi-Dirac function, q is the elementary charge, k is Boltzmann'sconstant, T is the temperature, φ_(t) is the trap level above the metaloxide (MoO₃) valence band maximum, v_(e) is the free electron velocity,N_(t) is the trap concentration, and P(V) is the tunneling probabilityover an interface barrier of height, φ_(B). Now,

$\begin{matrix}{{P(V)} = {\exp\lbrack {{- \frac{\propto}{E(V)}}\varphi_{B}^{\frac{3}{2}}} \rbrack}} & (2)\end{matrix}$where

$\alpha = \frac{4\sqrt{2m_{s}^{*}q}}{3h}$for a triangular energy barrier. Here, E(V) is the electric field atvoltage, V, m_(s)* is the electron effective mass in the organicsemiconductor, and h is the Planck's constant divided by 2π.

The exciton generation rate at current density J is:

$\begin{matrix}\begin{matrix}{{G(J)} = {\int{{G( {x,J} )}{\mathbb{d}x}}}} \\{= {\frac{1}{q}{\int{\frac{\mathbb{d}\lbrack {J_{e}(x)} \rbrack}{\mathbb{d}x}{\mathbb{d}x}}}}} \\{= {{- \frac{1}{q}}{\int{\frac{\mathbb{d}\lbrack {J_{h}(x)} \rbrack}{\mathbb{d}x}{\mathbb{d}x}}}}} \\{= {\frac{1}{q}J_{\gamma}}}\end{matrix} & (3)\end{matrix}$where G(x,J) is the volume generation rate of excitons between positionsx and x+dx in the EML, with x=0 taken at the EML/ETL interface. Theintegration is across the entire width of the EML. The charge balancefactor, γ, is the ratio of holes to electrons injected into the EML,given by:

$\begin{matrix}\begin{matrix}{\gamma = \frac{J_{h,A} - J_{h,C}}{J}} \\{= \frac{J_{c,C} - J_{c,A}}{J}}\end{matrix} & (4)\end{matrix}$

where J_(h,A), J_(h,C), J_(e,A), J_(e,C) are the hole (h) and electron(e) current densities at the anode (A) and cathode (C) sides of the EML.For high-efficiency electrophosphorescent OLEDs, the charge balancefactor is near unity, indicating that equal numbers of electrons andholes are simultaneously present in the recombination zone.

In a preferred aspect of the invention, the efficiency of the stackedOLED is improved by balancing the efficiency and charge injection acrosseach sub-element. Each sub-cell with the stacked device is preferablycharged balanced. In certain embodiments of the invention, the topelement of a stacked OLED having three sub-elements comprises aconventional cathode and a CGL (e.g. MoO₃/Li:BCP) anode, the middle OLEDcomprises a CGL for both cathode and anode, and the bottom elementconsists of a CGL cathode and ITO anode. To achieve equally highefficiency in each sub-element in the stack, different charge balancingmechanisms may be used for each element. These include (1) adjusting thethickness of the ETL, (2) inserting charge blocking layer(s) around theEML and (3) adjusting the thickness of the HTL.

Thus, in preferred embodiments of the invention, the charge balancefactor, γ, is near unity for each of the subcells in the stacked device.Preferably, the charge balance factor for each of the sub-cells is fromabout 0.9 to 1, and more preferably from about 0.95 to 1.

The optimized stacked WOLEDs of the present invention show a highefficiency. In preferred embodiments the stacked devices of the presentinvention have a total maximum external quantum efficiency of at leastabout 30%. More preferably, the stacked devices of the present inventionhave a total maximum external quantum efficiency of at least about 35%.In particularly preferred embodiments, the stacked WOLEDs of the presentinvention will have a high efficiency at a high brightness. Particularlypreferred stacked WOLEDs of the present invention have a total externalquantum efficiency of at least about 28% at a brightness of 1000 cd/m²,and more preferably at least about 32%.

The combined emission from the red, green and blue sub-elements give awhite emission from the stacked OLED. In preferred embodiments, thedevice is capable of emitting light having CIE coordinates ofX=0.37±0.08, and Y=0.37±0.08. More preferably, the device is capable ofemitting light having CIE coordinates of X=0.33±0.02, and Y=0.33±0.02.Moreover, the devices of present invention are preferably capable ofproducing a white emission having CRI of at least about 70. Morepreferably, the CRI is higher than about 75, and still more preferablythe CRI is higher than about 80.

It is understood that the various embodiments described herein are byway of example only, and are not intended to limit the scope of theinvention. For example, many of the materials and structures describedherein may be substituted with other materials and structures withoutdeviating from the spirit of the invention. It is understood thatvarious theories as to why the invention works are not intended to belimiting. For example, theories relating to charge transfer are notintended to be limiting.

Material Definitions and Abbreviations

-   CBP 4,4′-N,N-dicarbazole-biphenyl-   m-MTDATA 4,4′,4″-tris(3-methylphenylphenlyamino)triphenylamine-   Alq₃ 8-tris-hydroxyquinoline aluminum-   Bphen 4,7-diphenyl-1,10-phenanthroline-   n-Bphen n-doped BPhen (doped with lithium)-   F₄-TCNQ tetrafluoro-tetracyano-quinodimethane-   p-MTDATA p-doped m-MTDATA (doped with F₄-TCNQ)-   Ir(ppy)₃ tris(2-phenylpyridine)-iridium-   Ir(ppz)₃ tris(1-phenylpyrazoloto,N,C(2′)iridium(III)-   BCP 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline-   TAZ 3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4-triazole-   CuPc copper phthalocyanine-   ITO indium tin oxide-   NPD N,N′-diphenyl-N—N′-di(1-naphthyl)-benzidine-   TPD N,N′-diphenyl-N—N′-di(3-toly)-benzidine-   BAlq    aluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate-   mCP 1,3-N,N-dicarbazole-benzene-   DCM 4-(dicyanoethylene)-6-(4-dimethylaminostyryl-2-methyl)-4H-pyran-   DMOA N,N′-dimethylquinacridone-   PEDOT:PSS an aqueous dispersion of poly(3,4-ethylenedioxythiophene)    with-   polystyrenesulfonate (PSS)-   hfac hexafluoroacetylacetonate-   1,5-COD 1,5-cyclooctadiene-   VTES vinyltriethylsilane-   BTMSA bis(trimethylsilyl)acetylene-   Ru(acac)₃ tris(acetylacetonato)ruthenium(III)-   C₆₀ Carbon 60 (“Buckminsterfullerene”)-   FIr6 bis-(4′,6′-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate-   PQIr Ir(III)-bis-(2-phenylquinolyl-N,C^(2′))-acetylacetonate-   UGH2 p-bis(triphenylsilyly)benzene-   HTL hole transporting layer-   ETL electron transporting layer-   EML emissive layer-   CGL charge generating layer

EXPERIMENTAL

Specific representative embodiments of the invention will now bedescribed, including how such embodiments may be made. It is understoodthat the specific methods, materials, conditions, process parameters,apparatus and the like do not necessarily limit the scope of theinvention.

Example 1

The 20 Ω/sq indium tin oxide precoated glass substrates were degreasedin detergent solution and cleaned by solvents, followed by treatment forten-minute with UV/ozone before being transferred to a high vacuum of10⁻⁷ Torr. Then B, G, and R OLED sub-elements were sequentiallydeposited by thermal evaporation without breaking vacuum, with 10-nmBPhen layer doped with Li in a 1:1 molar ratio and a following 10-nmMoO₃ spaced in between each sub-element. Lithium, an n-type dopant here,is to add impurities to transfer electrons to LUMO states. For each OLEDsub-element, a 40-nm film of4,4′-bis[N-(l-naphthyl)-N-phenyl-amino]-biphenyl (NPD) as the HTL wasdeposited, followed by 25-nm EML, and then an ETL of 50-nm-thick BPhen.Here, BPhen, instead of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline(BCP), is used to reduce device drive voltage. Also note that in orderto maintain good charge balance at high bias when charge leakage is noteffectively prevented by energy barriers, thicker-than-normal (50 nm)BPhen layers are deposited.

Blue, green and red emissions are obtained from the phosphorescentdopants ofbis-(4′,6′-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6),tris-(phenylpyridine) iridium (Ir(ppy)₃), andIr(III)-bis-(2-phenylquinolyl-N,C^(2′))-acetylacetonate (PQIr),respectively. To optimize host materials for each of the dopants,dopant/host combinations are chosen asFIr6:p-bis-(triphenylsilyly)benzene (UGH2) for blue emission,Ir(ppy)₃:4,4′-N—N′-dicarbazole-biphenyl (CBP) for green,PQIr:4,4′-N,N′-dicarbazole-biphenyl (CBP) for red. Doping concentrationsare controlled between 8 wt % to 10 wt % for each cell. Finally, thecathode consisting of LiF (0.8 nm) and Al (120 nm) was deposited with ashadow mask to define a device area of 1.0 mm diameter.

The luminance at a fixed current density increases approximately as thesum of that for each independent OLED cell. FIG. 4 shows the schematicenergy level diagram of the R-G-B SOLED consisting of R, G, and B OLEDcells. The numbers indicate the respective HOMO and LUMO energiesrelative to vacuum (unit in eV). The HOMO and LUMO energies of Fir6,Ir(ppy)₃ and PQIr are (6.1 eV, 3.1 eV), (5.1 eV, 2.6 eV) and (5.0 eV,2.7 eV), respectively. The arrows indicate the carrier injection fromelectrodes and the MoO₃ charge-generation layers.

For balanced emission from each OLED element in order to achieve thedesired white color performance, it may be important to control the weakmicrocavity effects in the stacked device. To optimize the structure,therefore, the extraction efficiencies of the R, G, and B cells in thestacked structure are calculated based on transfer matrix simulations.The complex refractive indices of organics, ITO and MoO₃ employed in thesimulation are 1.7, 1.9-0.036·i and 1.9-0.3·i respectively. By varyingthe orders of three cells and the thickness of organic layers, thesimulation shows that the power extracted into air is a function of boththe wavelength and the source position (i.e., top, middle, and bottom,with bottom layer adjacent to ITO substrate). Results indicate that R,G, and B sub-elements, arranged in different orders, have differentextraction efficiencies and thus yield different color temperature andcolor rending indices (CRI) of the stacked device with other parametersstaying the same. By moving the three EMLs close to their correspondingoptical antinodes, the order of B-G-R (with R adjacent to the ITO anode)leads to the optimal color balance, with Commission Internationale deL′Eclairage (CIE) coordinates (0.39, 0.42) and a color rendering indexof CRI=79 at a current density of J=10 mA/cm², estimated to result in aluminance >1000 cd/m².

FIG. 5 shows the emission spectra for each sub-element as a function ofits position in the stacked structure, with cavity enhancement andextraction efficiencies considered. FIG. 11 shows the shows the powerextracted, considering microcavity effect as well as extractionefficiency, as a function of position and wavelength for the specificthickness of SOLED we use. With optimized doping concentrations, theconfiguration demonstrated above is one of the six order arrangement(RBG, RGB, etc) that gives the calculated Commission Internationale deL′Eclairage (CIE) coordinates and the CRI values (0.39, 0.42) and 79 ata current density of J=10 mA/cm², estimated to result in aluminance >1000 cd/m² under the assumptions that each cell has an IQEapproaching unity, and the CGLs are efficient in generating holes andelectrons.

FIG. 6 shows the experimental and simulated electroluminescence spectrafor the device under different current densities (J=1, 10, 100 mA/cm²).The CIE coordinates and the CRI values are (0.46, 0.36) and 61 at J=1mA/cm², and (0.36, 0.37) and 78 at J=100 mA/cm², respectively. Simulatedspectra are based on cavity enhancement and extraction efficiencies byfitting ratios of photons generated from each cell, with numbers shownin Table I. FIG. 7 compares the current density versus voltagecharacteristics of the stacked structure and control devices. The dashedline is the ideal J-V curve of the stacked device based on the J-V curveof the control devices without considering the voltage drop on the MoO₃layers. There exists excess drive voltage on the SOLED compared with thesum of that on all three control devices (solid line). This effect, dueto energy barriers at the CGL, accounts for a concomitant reduction of(10.3±0.7)% in power efficiency.

The external quantum and power efficiencies of the RGB SOLED, and themonochrome OLED control devices, measured in an integrating sphere, areshown in FIGS. 8 and 9. The blue, green and red controls exhibit EQEspeak at (13.9±1.0)%, (17.5±1.0)%, and (20.1±1.0)%, respectively. Thetotal EQE and power efficiencies for the RGB SOLED have maxima atη_(ext)=(36±2)% at a current density of J=82 μA/cm², and η_(p)=(21±1)lm/W at J=17 μA/cm², respectively. These values roll off to (32±2)% and(13±1) lm/W at 1000 cd/m² corresponding to J=2 mA/cm². The maximumexternal efficiencies of the RGB SOLEDs are approximately the sum of theEQEs of the three individual elements over a wide range of currentdensities, indicating that the losses at the transparent CGL areminimal. A fit of the SOLED EQE is shown in solid line, yielding anemission intensity ratio 0.7:0.5:1 in the B, G, and R elements. Thisdependence of exciton formation on position in the stack is attributedto the injection efficiencies of the CGLs and the ITO anode.

Table I provides the internal quantum efficiencies and the fraction ofphotons generated from the three stacked elements. As current densitiesincreases, we observe an increase of exciton formation on the blue andgreen elements with respect to that of the red element. This indicates acurrent dependent electron and hole injection efficiency from the CGLs.

TABLE I Internal quantum efficiencies and the ratio of photon generatedfrom R, G, and B cells in stacked device. Internal Ratio of quantumefficiency photon generated^((a)) J = 1 J = 10 J = 100 J = 1 J = 10 J =100 R (top) 0.69 0.67 0.48 1.00 1.00 1.00 G (middle) 0.84 0.65 0.42 0.450.64 2.22 B (bottom) 0.52 0.33 0.15 0.73 0.82 1.44 ^((a))Numbers are inarbitrary unit, normalized to the value of red devices under respectivecurrent densities.

Among the components of power efficiency for R-G-B SOLED, electricalefficiency, defined as V_(λ)/V, is important for improvement in powerefficiency. Note that V_(λ), is emissive photon energy in eV and V isthe operating voltage at a certain current density. The electricalefficiency of the SOLED drops from 0.14 at J=1 mA/cm² to 0.09 at J=100mA/cm², compare to 0.25, 0.38, and 0.28 for the B, G, and R controldevices, respectively, at the same current density. It can be increasedby improving charge transport, and charge injection particularly fromCGLs.

In summary, the R-G-B SOLEDs fabricated with all phosphorescent emittersand using a transparent CGL such as MoO₃/Li:BPhen show improved thedevice performance. White emission and SOLED efficiency were optimizedby making a tradeoff between the color emissive element ordering toachieve efficient charge injection and a maximum outcoupling efficiencyat a high CRI. The device reaches a maximum total external quantum andpower efficiencies of η_(ext)=(36±2)% and η_(p)=(21±1) lm/V,respectively. These results demonstrate electrophosphorescent RGB SOLEDsrepresent a promising architecture for achieving high brightness andefficiency for indoor lighting.

Example 2

To further understand and optimize the CGL architecture, the chargegeneration in CGLs based on transparent metal oxides was systematicallystudied. The current density-voltage (J-V) and capacitance-voltage (C-V)characteristics was analyzed for electron- and hole-only devicesconsisting of MoO₃ layers with varying thicknesses, and over a widerange of temperature. Optimized performance of a LiBCP/MoO₃ CGL isdemonstrated by varying both the thickness of MoO₃, as well as the Lidoping ratio in BCP. Thermally assisted tunneling from a trap level at(0.06±0.01)eV above the MoO₃ valence band maximum into the adjacentorganic layer is proposed to explain the temperature dependence of theJ-V characteristics in both electron- and hole-only devices.

Both the electron- and hole-only devices were prepared on detergent andsolvent cleaned glass substrates that were immediately transferred intoa vacuum chamber with a base pressure of 10⁻⁷ Torr after a 10-minuteexposure to a UV/ozone treatment. For the electron-only device shown inFIG. 13 a, a 50 nm-thick Al cathode to minimize hole injection wasdeposited onto the glass substrate through a 1 mm-wide striped shadowmask. This was followed by the deposition of a 40 nm-thick layer of BCPand a 10 nm-thick Li-doped layer of BCP in a 1:1 molar ratio. On thissurface, a layer of MoO₃, of different thicknesses (5, 10, and 20 nm)was deposited, followed by a second 50 nm-thick Al cathode depositedthrough 1-mm-wide striped shadow mask positioned perpendicular to theanode stripes. Similarly, for the hole-only device (see FIG. 13 b), a 50nm-thick Al electrode was deposited onto the glass substrate, followedby the deposition of a 10 nm-thick Li-doped BCP with 1:1 molar ratio,and MoO₃, of varied thicknesses (0, 5, 10, and 20 nm). Then a 40nm-thick 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl (NPD) layerwas deposited as a hole transport layer (HTL), followed by a 50 nm-thickMoO₃, electron blocking layer (EBL), and capped by a 50-nm thick Alcathode.

The ionization potentials and work functions used in the energy leveldiagrams have been taken from the literature. A work function of 5.7 eVwas used for MoO₃.

For characterization, samples were mounted in a cryostat where thetemperature was varied from 159K to 296K, and J-V characteristics weremeasured using a parameter analyzer (HP 4145B). The C-V measurementsemployed an impedance/gain-phase analyzer (HP 4194A), from which wasinferred the free carrier concentration and position of the interfacebarrier. The C-V measurements were obtained at a frequency of 200 Hz,which is sufficiently low to allow for dielectric relaxation. Opticalcharacterization of the devices employed a calibrated detector referenceusing standard methods described previously. (S. R. Forrest, D. D. C.Bradley and M. E. Thompson, Adv. Mater. 2003, 15, 1043).

The room temperature J-V characteristics of the electron-only devicewith MoO₃ thicknesses of 50 Å, 100 Å and 200 Å, are shown in FIG. 14 a.For electron injection, the Al electrode on the BCP side is positivelybiased relative to the Al electrode on the MoO₃ side. The lack ofrectification of the J-V characteristics indicates nearly equallyefficient electron injection from the CGL and the cathode. Under forwardbias (V>0), a dependence on MoO₃ thickness is observed, with 100 Å beingthe optimized thickness for the electron-only devices. FIG. 14 b showsthe hole-only device with no MoO₃ introduced, the current densities at agiven forward bias are reduced with the temperature ranging from 296K to159K, and a rectification ratio of ˜10⁴ at ±5 V is observed at roomtemperature.

The C-V characteristics of the electron-only devices are shown in FIG.15 a. Depletion layer widths calculated from the capacitance are shownin FIG. 15 b. In FIG. 15 c, the effect of different free carrierconcentrations on interface depletion width is demonstrated for CGLswith a 100 Å-thick layer of MoO₃. The concentration of Li in BCP isvaried from 1:1 to 1:10 molar ratio, corresponding to depletion widthsof 24 Å and 85 Å, respectively. The device without Li doping shows afully depleted region with a thickness of 110 Å.

The current densities as functions of 1/E(V) for various temperaturesranging from 159K to 296K are plotted for electron-only (FIG. 16)devices shown in FIG. 13 a. Here, the electric field is taken as theratio of the applied voltage to the charge generation layer thickness,after subtracting the 2.7V built-in potential. Small voltage drops atthe contact/organic layer interface and across the highly Li-dopedlayers are neglected. In FIG. 17, the current densities J vs. 1000/T areplotted for an electric field E=2.0×10⁷ V/cm, which we obtain the trapactivation energy φ_(t). Corresponding plots for hole-only devices areshown in FIGS. 18 and 19, respectively.

The presence of a MoO₃ layer is important for efficient chargegeneration, as shown by comparison of FIGS. 14 a and 14 b. Under reversebias, both Al contacts are nearly ohmic due to the high Li concentrationin BCP, as well as due to the 50 Å-thick MoO₃ between the NPD and the Alcathode which, in combination, enhance hole injection. Under forwardbias, however, both electron and hole injection are reduced at theelectrodes, and the current density is a result of carrier generationfrom the CGL. With efficient injection and transport facilitated forboth electrons and holes under both reverse and forward biases,symmetric J-V characteristics are observed for electron-only deviceswith various MoO₃ thicknesses, as shown in FIG. 14 a. Among the CGLswith varied thicknesses of MoO₃, the device with a 100 Å-thick CGL showsa high generation efficiency, with a current three to four times higherthan for 50 Å and 200 Å-thick MoO₃ layers at >2V under forward bias. TheMoO₃ is too thin to result in complete and uniform coverage at 50 Å,hence reducing injection at this interface, while at thicknesses >100 Å,tunneling injection is significantly attenuated.

The J-V characteristics of the hole-only devices yield a similardependence on MoO₃ thickness. Shown in FIG. 14 b are the J-Vcharacteristics of a hole-only device without MoO₃, with the structureAl (500 Å)/Li:BCP (100 Å)/NPD (400 Å)/MoO₃ (50 Å)/Al (500 Å). Here the50 Å-thick MoO₃ adjacent to Al cathode acts as an EBL. Inefficientcarrier generation was observed under forward bias due to the absence ofMoO₃, resulting in a rectification ratio of ˜10⁴ at ±5V at roomtemperature. The hysteresis behavior at 159K, shown in FIG. 14 b wherezero current occurs at −1.2V for voltage swept from −5 to 5V, ispossibly due to electron capture and delayed re-emission at defectstates in MoO₃ introduced during film deposition.

To understand the thickness dependence of the charge carrier generationefficiency, C-V measurements for the electron-only devices are shown inFIG. 15 a. The depletion widths in the doped BCP layer, are 30 Å, 24 Åand 26 Å for CGLs with MoO₃ thicknesses of 50 Å, 100 Å and 200 Å,respectively (FIG. 15 b). The relative static permittivity used todetermine the carrier concentration is 3.0 for the organic layers. Inthe case of 1:1 Li:BCP, the electron concentration in BCP is calculatedto be N_(d)=10¹⁹˜10²⁰ cm⁻³ as inferred from the depletion width of 24 Å.This is in agreement with the 1:1 molar ratio of Li:BCP dopingconcentration, suggesting one electron per Li atom. The estimation ofN_(d) is significantly larger than previously reported (˜10¹⁸ cm³) fromconductivity measurements, where the formation of a BCP-Li complex wassuggested to explain the difference of Li doping concentration and thecarrier density of the doped film. The doped BCP layer ensures a verythin depletion layer that allows for efficient electron injection. Sincethe tunneling probability is an exponential function of tunnelingdistance, the 100 Å-thick MoO₃ sample results in the highest tunnelinginjection efficiency compared to the other thicknesses used.

To extract energy barrier φ_(B), the J vs. E⁻¹ characteristics of theelectron-only devices with various MoO₃ thicknesses are plotted in FIG.16, where E is calculated by subtracting the built-in potential, 2.7V,from the applied voltage. Since Li:BCP and MoO₃ are highly doped n-typeand p-type semiconductor materials, respectively, the built-in potentialat the Li:BCP/MoO₃ junction is determined by the difference between BCPLUMO (3.0 eV) and MoO₃ valence band maximum (5.7 eV). Linearrelationships in log(J) vs. E⁻¹ are observed for devices in thetemperature range from 296K to 159K. Energy barriers, φ_(B), obtainedfrom the fit of Eq. (1) to these data are listed in Table II.

To extract the trap activation energy, φ_(t), the current densities Jvs. 1000/T for these same data are plotted in FIG. 17. The slopes of thefits (solid lines) yield φ_(t)=(0.06±0.01)eV independent of MoO₃thickness. The intercepts yield the value, qv_(e)N_(t) 10⁶ A/cm². Takingthe electron thermal velocity of v_(e) 10⁷ cm/s, we obtain a trapconcentration N_(t)˜10¹⁸/cm³, as listed, in Table II.

Similar plots for hole-only devices are shown in FIGS. 18 and 19. Theelectric field within the CGL is more complicated to estimate than forelectron-only devices due to the voltage drop across the undoped NPD.Hence, we fabricated the following device: ITO (1500 Å)/NPD (400 Å)/MoO₃(100 Å)/Al (500 Å) to determine the E. From these data, we obtain φ_(B)and φ_(t), with the results also presented in Table II. Agreementbetween the energies and trap densities obtained for both the electron-and hole-only devices provides significant support for this model.

TABLE II Tunneling barrier, φ_(B), and trap depth, φ_(t), and trapdensity, N_(t), of electron- and hole-only devices with MoO₃ thickness.Devices 50 Å l00 Å 200 Å electron-only φ_(B) (eV) 1.1 ± 0.1 1.3 ± 0.11.2 ± 0.1 φ_(t) (eV) 0.07 ± 0.01 0.06 ± 0.01 0.06 ± 0.01 Nt (×10⁻¹⁸cm⁻³) 1.2 ± 0.8 12.5 ± 7.3  2.5 ± 1.4 hole only φ_(B) (eV) 1.1 ± 0.1 1.0± 0.1 1.0 ± 0.1 φ_(t) (eV) 0.08 ± 0.02 0.09 ± 0.02 0.09 ± 0.02 Nt(×10⁻¹⁸ cm⁻³) 15.7 ± 9.2  9.9 ± 5.8 3.1 ± 1.9

Example 3

To determine the effects of the charge generation efficiency on theperformance of a green emitting SOLED with more than two sub-elements,OLEDs using the CGL as either a cathode (Cell-L), an anode (Cell-R), orboth (Cell-M) were fabricated (see FIG. 20 a), as well as the controldevice with an ITO anode/Al cathode combination. Detailed structures areprovided in Table III. Note that for Cell-R and Cell-M, 20 Å-thick Alwas directly deposited onto ITO to ensure band alignment at themetal/organic interface, and thus to decrease the significant energybarrier that prevents electron transport from the CGL to ITO.

TABLE III Structure of the subcells in a 3 layer SOLED, and the controlOLED. Devices Layer Functions Materials Thicknesses (Å) Cell-L anode ITO1500 HTL NPD 400 EML Ir(ppy)₃:CBP 250 ETL BCP 500 CGL Li:BCP/MoO₃100/100 cathode Al 500 Cell-M anode ITO/Al 1500/20  CGL Li:BCP/MoO₃100/100 HTL NPD 400 EML Ir(ppy)₃:CBP 250 ETL BCP 500 CGL Li:BCP/MoO₃100/100 cathode Al 500 Cell-R anode ITO/Al 1500/20  CGL Li:BCP/MoO₃100/100 HTL NPD 400 EML Ir(ppy)₃:CBP 250 ETL BCP 400 cathode LiF/Al 8/500 control anode ITO 1500 HTL NPD 400 EML Ir(ppy)₃:CBP 250 ETL BCP400 cathode LiF/Al  8/500

The EQE and PE of each device are shown in FIGS. 20 b and 20 c. Thecontrol device shows a peak forward-viewing EQE=(8.9±0.2)% at currentdensity J=0.13 mA/cm², similar to previously reported Ir(ppy)₃-basedelectrophosphorescent OLEDS. A peak forward viewing EQE=(10.5±0.2)% isobserved for Cell-L at J=0.37 mA/cm², and EQE=(10.6±0.2)% at J=39 μA/cm²for Cell-M. In contrast, Cell-R shows a significantly reduced peakEQE=(5.3±0.2)% at a current density of J=0.92 mA/cm². The PE forCells-L, -M, and -R have maxima of (26±1), (29±1), and (15±1) lm/W,respectively, compared to PE=(23±1) lm/W for the control device.

To achieve high efficiency and brightness, CGLs preferably also providefor charge balance in each emitting element when used as connectingelectrodes in SOLEDs, as suggested by Eqs. (3) and (4). For devices inFIG. 20 a, no NPD emission is observed as a function of current density,and hence we infer that the devices are hole-rich. The improved electrontransport to the EML achieved by the CGL in Cell-L leads to an enhancedEQE. In contrast, hole leakage through Cell-R results in chargeimbalance and an EQE, that is considerably less than that of Cell-L, -Mand the control OLED. Note that optical interference effects introducedby CGLs and the thin Al layers in all three cells have been calculatedbased on transfer matrix simulations, leading to only a small (3%)effect on the power efficiencies, and hence cannot be the cause of thereduced EQE of Cell-R.

Comparing the efficiency of the control device with those of eachsub-element in the stack, we obtain the following charge balancefractions for Cells-L, -M, and -R (c.f. FIG. 20 a):

$\begin{matrix}\begin{matrix}{\gamma_{{Cell}\text{-}L} = \frac{J_{h,L} - J_{h,L}^{\prime}}{J_{h,L}}} \\{= \frac{10.5\%}{{EQE}_{\max}}}\end{matrix} & ( {5a} ) \\\begin{matrix}{\gamma_{{Cell}\text{-}M} = \frac{J_{h,M} - J_{h,M}^{\prime}}{J_{h,M}}} \\{= \frac{10.1\%}{{EQE}_{\max}}}\end{matrix} & ( {5b} ) \\\begin{matrix}{\gamma_{{Cell}\text{-}R} = \frac{J_{h,R} - J_{h,R}^{\prime}}{J_{h,R}}} \\{= \frac{5.1\%}{{EQE}_{\max}}}\end{matrix} & ( {5c} )\end{matrix}$

Under charge neutrality at both electrodes, we have:J _(h,L) =J _(e,R) +J _(h,R) =J _(max)˜0.4 mA/cm²  (5d)

Equations (5a-5d) then give γ_(cell-L)=1, γ_(Cell-M)=0.96, andγ_(Cell-R)=0.49, suggesting nearly unity charge balance in the twoformer cells. However, in Cell-R, there is a large hole-currentimbalance J_(h,R)=0.19 mA/cm², whose presence results in thesignificantly reduced EQE of that sub-element.

To optimize Cell-R, the hole current was controlled by using variousthicknesses of BCP, ranging from 300 Å to 600 Å. As shown in FIG. 21,peak EQEs of (3.7±0.2)%, (5.1±0.2)%, (8.3±0.2)%, and (8.6±0.2)% areobserved for BCP thicknesses of 300 Å, 400 Å, 500 Å, and 600 Å,respectively. The corresponding power efficiencies have maximum valuesof (11±1) lm/W, (15±1) lm/W, (24±1) lm/W, and (22±1) lm/W. IncreasedEQEs and PEs are observed for the devices with BCP thicknesses of 500 Åand 600 Å. Thus, by changing only the transport layer thickness (andhence its resistance), we can significantly improve cell efficiency,which supports the conclusion that charge imbalance in Cell-R is theprimary mechanism for efficiency loss. Of the various means of achievingcharge balance, ohmic hole and electron injection into the EML may beoptimal. Hence, employing charge blocking layers as opposed toincreasing layer resistance (as done here) provides the highestcombination of PE and EQE for each element in the stack.

The EQEs and PEs of the G-G-G SOLEDs with varied BCP thicknesses inCell-R, from 400 Å to 600 Å, are shown in FIGS. 22 a and 22 b,respectively. Devices with 300 Å, 400 Å, 500 Å, and 600 Å-thick BCPexhibit forward-viewing EQEs peaking at (20.5±1.0)%, (21.6±1.0)%, and(24.3±1.0)%, and (23.1±1.0)%, respectively, at a current density ofJ=1.4×10⁻⁴ A/cm². The optimized G-G-G SOLED, with 500 Å-thick BCP inCell-R, shows a peak forward-viewing PE=(19±1) lm/W at the currentdensity of J=1.7×10⁻⁵ A/cm², which, rolls off to (12±1) lm/W at 1000cd/m² corresponding to J=1.2×10⁻³ A/cm². The EQEs of the G-G-G SOLEDsare approximately the sum of the EQEs of the three individual OLEDs overa wide range of current densities, indicating that the losses at thetransparent CGL are minimal.

While the present invention is described with respect to particularexamples and preferred embodiments, it is understood that the presentinvention is not limited to these examples and embodiments. The presentinvention as claimed therefore includes variations from the particularexamples and preferred embodiments described herein, as will be apparentto one of skill in the art.

We claim:
 1. An organic light emitting device comprising in order: acathode; a blue-emitting sub-element comprising an emissive layercomprising a phosphorescent blue emissive material; a charge-generatinglayer; a green-emitting sub-element comprising an emissive layercomprising a phosphorescent green emissive material; a charge-generatinglayer; a red-emitting sub-element comprising an emissive layercomprising a phosphorescent red emissive material; and an anode; whereina combined emission of the emissive materials gives a white emissionfrom the device.
 2. The organic light emitting device of claim 1,wherein each sub-element comprises a hole transporting layer, anelectron transporting layer and the emissive layer, wherein the emissivelayer is the hole transporting layer, the electron transporting layer ora separate layer.
 3. The organic light emitting device of claim 1,wherein the device emits light having CIE coordinates of X=0.37±0.08,and Y=0.37±0.08.
 4. The organic light emitting device of claim 1,wherein the device is capable of a maximum external quantum efficiencyof at least about 30%.
 5. The organic light emitting device of claim 1,wherein the charge-generating layers comprises an n-type layer adjacentto a p-type layer.
 6. The organic light emitting device of claim 5,wherein the p-type layers comprise a material selected from MoO₃, V₂O₅,ITO, TiO₂, WO₃ and SnO₂.
 7. The organic light emitting device of claim6, wherein the p-type layers comprise MoO₃.
 8. The organic lightemitting device of claim 1, wherein the emissive layers of eachsubelement are within about 20% of the distance to an antinode for thatemitter.
 9. The organic light emitting device of claim 1, wherein atleast two of the emissive layers are close enough to an antinode so asto provide at least 90% of its maximum emission.
 10. The organic lightemitting device of claim 9, wherein each of the emissive layers areclose enough to an antinode so as to provide at least 90% of its maximumemission.